Field effect transistors (FETs) are transistors in which the resistance of the current path from source to drain is modulated by applying a transverse electric field between grid or gate electrodes. The electric field varies the thickness of the depletion layer between the gates, thereby modifying the conductance. Organic field effect transistors (OFETs) utilize an organic semiconductor channel, such as polythiophene compounds, in place of conventional inorganic semiconducting materials. An OFET as generally practiced in the prior art is depicted in FIG. 1, where a gate electrode 2 is situated on a substrate 1, a gate dielectric layer 3 is disposed on the gate electrode 2, an organic semiconductor layer 4 used as an active layer of the transistor is formed on the gate dielectric layer, and source and drain electrodes 5 and 6 are formed on the organic semiconductor layer 4. The gate electrode 2 is typically formed in the organic transistor forming region by depositing a gate metal such as, for example, Cr/Au or Ti/Au and the thickness of the gate electrode 2 is typically about 1000 Ångstroms. On the gate electrode 2 in the transistor region, a dielectric layer 3 that insulates the gate electrode from other members is made of a non-conducting substance and is formed by a vacuum evaporation or a spin coating method with a nominal thickness of 3 micrometers and a conductivity less than 10−14 ohm/cm. The organic semiconductor layer 4 used as an active layer of the transistor is deposited by a spin coating or vacuum deposition method on the gate-insulating layer 3. Preferably, the thickness of the organic semiconductor layer 4 is less than 100 nm. The organic semiconductor layer 4 of the OFET can be made of a charge transfer complex or a thiophene polymer in order to enhance the mobility and the driving current of the field effect transistor. By way of example, the charge transfer complex is selected from the group consisting of copper phthalocyanine, tetrametyltetraselennafulvalene, bis (tetra-n-butylammonium) palladium (II), tetrathiafulvalene, and 7,7,8,8-tetracyano-p-quinodimethane. Then, a gold film with high electrical conductivity is deposited and the deposited gold film is etched so as to form a source electrode 5 and a drain electrode 6.
The interfaces in an OFET are critical to its performance. Carrier injection from the source and drain electrodes into the organic semiconducting layer is a function of the intimacy of the contact. Similarly, the interface created between the dielectric and semiconducting layer must be planar, uniform, oriented, promote adhesion, and/or act as a coupling agent. Traditional interfaces consist of gold, silver, copper, palladium, platinum, and other noble metals, in contact with the organic semiconductor. These metal-semiconductor interfaces lack optimal complementary surface energies to permit totally intimate contact. This results in poor carrier injection, lowered mobility, and poor device performance, due in part to the lack of either chemical bonding or surface wetting. If the quality of these prior art interfaces could be improved, then an OFET with substantially improved performance could be realized.